Abstract: The disclosed system of charging a battery provides for accurate regulation of the charging current which is maintained constant during the initial and final phases of the charging cycle. The slope of battery-voltage increase during continued charging of the battery is monitored at frequent intervals. Failure of the battery voltage to rise more than a threshold increment during a slope-testing interval signifies full-charge, and is utilized to terminate the charging operation. The disclosed slope detector includes a storage capacitor having a terminal that is isolated ideally from leakage-causing circuit connections.The disclosed circuit is economical in terms of low cost and avoidance of complexity, and includes provision for suppressing potentially dangerous voltages at the battery terminals when there is no connected battery and for blocking charge of a battery connected backwards and of a battery whose voltage is so low as to signify a defect rather than depleted condition.